Few apparatuses have been developed for the treatment of arm lymphedema. The objective of this study was to use bioimpedance to evaluate the efficiency of the RAGodoy® mechanical drainage device in reducing swelling in the treatment of mastectomy-related arm lymphedema. Twenty-one patients with arm lymphedema after mastectomy were enrolled in a prospective study (clinical trial) to quantitatively evaluate reductions in limb size using a passive electromechanical device to perform mechanical lymph drainage. The study was conducted in the Vascular Laser Center in Sao Jose do Rio Preto. The InBody S10® body composition analyzer was used to evaluate edema. The paired t-test was used for statistical analysis with significance being set for an alpha error = 5% (p-value < 0.05). The results showed that a significant reduction in edema was observed after mechanical lymph drainage using the RAGodoy® device (p-value < 0.012). In conclusion, lymph drainage performed with the electromechanical RAGodoy® device is effective in reducing volume of arm lymphedema as assessed by bioimpedance.